The present invention relates to a method and apparatus for creating complex, unbalanced and non-uniform shaped structures.
A wide variety of products in the office furniture industry are presently manufactured which can be described as having complex, unbalanced and non-uniform shapes. Chairs, tables and the like employ parts with shapes dictated, in part, by structural as well as aesthetic and ornamental considerations incorporated into the article by the designer. Chair arms, for example, must have sufficient strength to provide adequate arm support. The arms also perform an ornamental function and, hence, may have a complex, non-uniform shape.
Heretofore, complexly configured structures have been manufactured using a variety of techniques. The parts may be cast from metal or molded from engineering plastics. Casting techniques suffer from inherent problems related to cost, material waste, labor and quality control. Porosity and material shrinkage, for example, may result in shape changes, surface defects and the like. Generally, complexly configured cast articles must have additional machining operations performed on them so that the desired shape and tolerances are obtained.
In many instances, configured structural parts such as automotive framing and chair components may be formed by welding together sheet metal stamped parts. The resulting structures have a closed, looped configuration in cross-section and provide the needed strength without many of the disadvantages of casting processes. Welding of the stamped parts can, however, cause heat warpage and shape changes. Weld splatter can have an adverse effect on surface finish. In addition, blanking and trimming operations associated with stamped parts result in significant process scrap.
The use of tubing having a closed loop cross-section which may be circular, oval or rectangular to form structural parts has been proposed. Tubing can be bent, flared, swedged, drawn and sized and, hence, formed into a wide variety of shaped parts. The specific shapes and areas where tubing could be used have been inherently limited, however, by known manufacturing processes. Formation of tubing into various shapes through bending operations may be limited by shape changes caused by uncontrolled weld seam location, heat distortion caused during welding operations and material springback. Springback, that is, the elastic recovery of the material from which the part is made, causes the shaped part to move back toward an original prebent configuration.
Various processes have been proposed to form parts or to bend tubing to compensate for such springback and material deformation. An example of one such method and apparatus may be found in U.S. Pat. No. 3,821,525 entitled METHOD AND APPARATUS FOR AUTOMATICALLY COMPENSATED TUBE BENDING and which issued on Jun. 28, 1974, to Eaton et al. As set forth therein, in order to bend a tube to a desired bend angle, it was necessary to "overbend" the tube initially so that, when released, it would spring back to the desired bend angle. Springback makes maintenance of required tolerances and the part configuration extremely difficult. Normal tube bending processes required incremental steps or stages and required the tube or bending tools to be moved during the forming process. Portions of the tube between bend points tended to return to original straight conditions. In order to compensate for springback, a machine is disclosed in the aforementioned patent which includes a carriage assembly having a rotating collet chuck for selectively gripping the end of a length of tubing. The tube is longitudinally moved with respect to a bend die. The tube is moved to the desired position by the carriage and a clamp die and bent. A wiper die and pressure die grip a portion of the tube behind the bend to provide controlled elongation of the tube around the bend. An automatic controller or computer and sensors indicate the position of the bend die, carriage and collet chuck. The controller generates operating commands and receives inputs from conventional sources. The system attempts to predetermine the proper degree of overbend and fixed and radial compensation to provide a tube having a desired shape.
An example of another method and apparatus is disclosed in U.S. Pat. No. 4,854,150 entitled METHOD OF BENDING AND FORMING HEATED TUBULAR WORKPIECES and issued on Aug. 8, 1989 to Brown et al. The method and apparatus disclosed therein forms a tube into a desired shape through controlled heating, forging and bending followed by quenching. The process results in a permanent form. A tubular bar or workpiece is heated and held against a forming anvil by clamping structure. The heated tubular bar is bent by a plurality of forming rollers around the anvils. An hydraulic system and a plurality of actuators control the positioning of the forming rollers. Heating and quenching steps are used for configuration stability.
A need exists for a method and apparatus which is capable of forming or creating complex, unbalanced and non-uniform shaped structures from tubing in an efficient, cost effective manner and which results in stable parts having complete shape retention, acceptable tolerances and no memory or springback.